Typically, superabsorbent polymers (SAP's) have been prepared by gel polymerization of water-soluble monomers in aqueous solution. Certain additives, such as cross linking agents, may be incorporated into the monomer mixture. The product of the polymerization process is then typically dried and subjected to mechanical means of particle size reduction and classification including chopping, grinding, and sieving. Optionally, the product may be subjected to a post-treatment to improve its quality and performance, especially its ability to absorb aqueous fluids. Such post-treatments include surface post-crosslinking, heat treatment and heat treatment in the presence of an oxidizing agent, such a sodium or potassium chlorate.
Superabsorbent polymers are primarily used in personal care products which absorb body fluids, for example, baby diapers, adult incontinence products and feminine hygiene products. In these applications, SAP particles are incorporated into absorbent structures which contain, for example, synthetic and natural fibers which may or may not be in the form of paper based woven and non-woven structures, and toughened masses of fibers, such as fluff pads. The materials used in such structures can rapidly acquire aqueous fluids, distribute the fluids over the whole absorbent structure and absorb and retain them. The absorbent structures, in the absence of SAP particles, have limited absorption capacity, are bulky due to the large amount of material needed to provide acceptable absorption capacity, and inadequately retain fluid under pressure.
However, there is a trend to design absorbent structures having various SAP concentrations distributed in the absorbent structure in order to achieve a desired absorbency profile of the absorbent device. See, for example, WO 95/26209. It is also a trend to apply two or more SAP's which differ in their absorption characteristics and to distribute them in specific patterns within the structure. See, for example, WO 95/01146. In such structures, it is essential that the SAP(s), distributed in such complicated patterns, stays in the desired region, especially under use conditions. Furthermore, it would be desired to prepare superabsorbent polymers which could be easily shaped into various different forms and fixed onto the relevant structure in any desired concentration. With the prior art granular SAP's, as described above, such goals would only be achievable through burdensome handling and fixing of the ground SAP particles and economically unfeasible manufacturing processes for the fixing application of the SAP particles to the absorbent structures.
Thus, alternative manufacturing processes for superabsorbent polymers and even new superabsorbent polymers which could be employed in absorbent structures having a specific absorbency profile are sought.
U.S. Pat. No. 3,926,891 refers to water-swellable polyacrylate articles made from a solution of the polyacrylate having an effective amount of a soluble crosslinking agent therein by heating and/or drying the solution. The polyacrylate solution is made from a polyacrylate by saponification and the crosslinking agent is then added.
U.S. Pat. No. 4,071,650 and U.S. Pat. No. 4,076,928 refer to water-swellable articles made from copolymers having a copolymerized crosslinker, methods for preparing such articles, as well as to compositions containing a copolymerized crosslinker useful to make said articles. The articles are crosslinked by heating and/or removing substantially all of the water from the precursor composition.
U.S. Pat. No. 4,117,184 is directed to water-swellable aerated films and laminates made from solutions of carboxylic polyelectrolytes and methods for preparing same. The films and laminates are cured and/or crosslinked with a polyfunctional or difunctional crosslinking agent that is reactive with carboxylate groups by heating and/or removing substantially all of the water and/or alcohol from the precursor composition. The solutions are mechanically aerated prior to the curing step.
U.S. Pat. No. 4,339,371 refers to water-in-oil emulsions containing high concentrations of water-soluble polymers which are prepared by incorporating therein an oil-soluble, water-insoluble polymeric surfactant prepared from maleic anhydride and a comonomer.
AU-A-10105/88 describes the manufacture of noncrosslinked poly(acrylic acid) and its salts using an inverse water-in-oil emulsion polymerization process that requires a co-water-in-oil emulsifier which is a C12-C22 fatty acid, and preferably is oleic acid.
WO 93/18223 teaches a process for treating a substrate with a superabsorbent material in which process a layer of a water-in-oil SAP-containing emulsion, containing the superabsorbent material in its aqueous phase, is applied to the substrate in such a manner that 0.3 to 40 weight percent, calculated on its dry weight, of the superabsorbent material is applied to the substrate. After this step, the liquid constituents of the emulsion are wholly or partially removed from the substrate. Only crosslinked materials are employed in this patent application. Disclosed examples of superabsorbent materials include crosslinked polyacrylic acid partially neutralized into the sodium salt. The teachings of this reference do not cover any film formation, homogenization of the spread to form a homogeneous gel matrix, or post-crosslinking. Furthermore, the articles made in accordance with the teachings of this reference provide a high gel blocking behavior.
Korean Patent 915 506 describes the preparation of a water absorbable resin in the form of flakes from a crosslinked, partly neutralized polyacrylic acid which is provided in the form of a water-in-oil emulsion. After partly drying and shaping (flattening) of the polymer emulsion with the aid of a drum dryer, the surface of the flakes obtained is post-crosslinked with diglycidyl ethers.
None of these patents or patent applications, however, describes a process for preparing a SAP of desired properties from highly concentrated precursor polymers which have a high enough molecular weight and which are easily applicable to absorbent structures whereby the concentration, pattern and form of the SAP distribution can be easily designed. Therefore, there still remains the need for such a process and the SAP's resulting therefrom.